The invention relates to light-emitting quantum well semiconductor devices with improved spontaneous photon emission.
For light emitting diodes based on spontaneous emission, competing mechanisms for carrier recombination limit the light output efficiency. Non-radiative recombination of charge carriers injected at a p-n junction in a semiconducting device, for example, may occur in the semiconducting device at a faster rate than radiative recombination. In many cases, therefore, it would be desirable to increase the radiative recombination rate in the semiconductor device to compete more effectively with non-radiative mechanisms. Also, the spontaneous emission in a large optical cavity typically takes place in 4.pi. steradians of space, that is in all directions, and again not all emitted light is collected. In some cases, therefore, it is desirable to cause the spontaneous radiation emitted in a light emitting diode to be emitted in a preferred direction.
Vertical-cavity surface-emitting lasers (VCSEL's) using a single, or only a few, quantum wells (QW's) are well known in the art. Such lasers have short cavities with highly reflecting end mirrors (reflectivities on the order of 0.998), in order to ensure lasing and to promote stimulated photon emission in the quantum well. The light emission in the VCSEL and the optical cavity occurs normal to the epitaxial crystal growth surface. The short gain length of these known VCSEL's place stringent requirements on the mirror reflectivities in order that the round trip gain-length product overcome cavity losses which arise due to mirror reflectivities having values less than unity, and also other parasitic cavity losses. The preferred method of realizing the highly reflecting mirrors makes use of pairs of alternating semiconductor materials of differing refractive index to form a distributed Bragg reflector (DBR). Because of typically small index ratios between semiconductor materials, a large number of pairs (N.sub.pairs &gt;23) is required to achieve the desired reflectivity value for a single QW VCSEL of .about.0.998. Semiconductor DBR mirrors are in many cases preferred for the QW VCSEL because they can be made electrically conducting. However, other mirror forms based on evaporated dielectrics and/or metals have also been used. The previously perceived strict requirement on mirror reflectivity for lasing has thus far limited device optical output efficiency due to the inability to achieve lasing when the reflectivity product of both end mirrors falls below a specified value, which for the single QW VCSEL is on the order of 0.997. Current QW VCSEL devices base the required mirror design in terms of mirror reflectivity on known values of the optical gain achievable from a semiconducting QW in an optical environment resembling an infinite cavity, and which has not been specifically designed to exert an influence over the QW spontaneous emission.
While many VCSEL's may currently benefit to some degree from a cavity enhanced gain, the enhancement has not previously been recognized nor taken advantage of in device design. Instead, current teaching assumes that QW gain in a VCSEL is unchanged over its value in a larger optical cavity, for example unchanged over that of more standard edge-emitting, semiconductor lasers. Thus, prior end mirror reflectivities for laser cavities are designed to achieve reflectivity values as governed by the equation: EQU R.sub.1 R.sub.2 .gtoreq.e.sup.-gmax . Lz,
where R.sub.1 and R.sub.2 are the end mirror reflectivities, g.sub.max is the gain per unit length of the active region (preferably a single QW), and L.sub.z is the length of the active region. For a typical prior VCSEL design, L.sub.z is on the order of 80 .ANG., g.sub.max is assumed to be approximately 4.times.10.sup.3 cm.sup.-1, which results in the above-noted reflectivity product of greater than or equal to 0.997 in order to achieve lasing.
It would therefore be desirable to provide a light-emitting semiconductor device which exploits the advantages of the QW structure in diodes and in the VCSEL, while relaxing the requirement for highly reflecting mirrors, and thus improve device performance in terms of improved lasing efficiency, reduced electrical series resistance, or increased flexibility in device geometry.